Technical Field
The present disclosure relates to a liquid crystal display including an optical film embedded therein. In particular, the present disclosure relates to a liquid crystal display having a structure, in which an optical film for uniformity and concentration of light provided by a backlight unit is attached to a lower polarizer, and a method of manufacturing the liquid crystal display.
Description of the Related Art
A range of application for liquid crystal displays has gradually widened because of its excellent characteristics such as light weight, thin profile, and low power consumption. The liquid crystal displays have been used in personal computers such as notebook PCs, office automation equipments, audio/video equipments, interior/outdoor advertising display devices, and the like. A transmissive liquid crystal display occupying most of the liquid crystal displays controls an electric field applied to a liquid crystal layer and modulates light incident from a backlight unit, thereby displaying an image.
A backlight unit is classified into a direct type backlight unit and an edge type backlight unit. The direct type backlight unit is configured such that a plurality of light sources is disposed under a liquid crystal display panel. The edge type backlight unit is configured such that light sources are disposed opposite the side of a light guide plate, and a plurality of optical films is disposed between a liquid crystal display panel and the light guide plate. In the edge type backlight unit, the light source irradiates light onto one side of the light guide plate, and the light guide plate converts linear light or point light into surface light. The edge type backlight unit has an advantage over the direct type backlight unit of being thinner.
A liquid crystal display including an edge type backlight unit according to a related art is described below with reference to FIGS. 1 and 2. FIG. 1 is an exploded perspective view illustrating a structure of a liquid crystal display including an edge type backlight unit according to a related art. FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1.
Referring to FIGS. 1 and 2, a related art liquid crystal display includes a liquid crystal display panel LCP and an edge type backlight unit EBLU disposed below the liquid crystal display panel LCP. The liquid crystal display panel LCP includes an upper glass substrate SU, a lower glass substrate SL, and a liquid crystal layer LC between the upper and lower glass substrates SU and SL and may be implemented in any liquid crystal mode.
The edge type backlight unit EBLU includes a light source LS, a light guide plate LG, and an optical film OPT. The edge type backlight unit EBLU converts light emitted from the light source LS into uniform surface light through the light guide plate LG and the optical film OPT and provides the liquid crystal display panel LCP with the uniform surface light. The edge type backlight unit EBLU may further include a reflective plate REF, that is positioned below the light guide plate LG and returns light leaked through a lower surface of the light guide plate LG to the light guide plate LG.
A cover bottom CB is disposed below the reflective plate REF. It is preferable, but not required, that the cover bottom CB has a container shape for containing the edge type backlight unit EBLU therein. Further, the cover bottom CB includes a material with high heat conductivity and high rigidity, in order to smoothly dissipate heat from the light source LS to the outside. For example, the cover bottom CB may be made of a metal plate, such as aluminum (Al), aluminum nitride (AlN), electrolytic galvanized iron (EGI), steel use stainless (SUS), galvalume steel coil (SGLC), aluminum coated steel (ALCOSTA), and steel plated tin (SPTE). Further, a material with high conductivity for accelerating heat transfer may be coated on the metal plate.
A guide panel GP and a top case TC are disposed at an edge of the liquid crystal display panel LCP. The guide panel GP is a rectangular mold frame, in which a glass fiber is mixed in a synthetic resin, for example, polycarbonate. The guide panel GP surrounds an upper edge and a side of the liquid crystal display panel LCP and a side of the edge type backlight unit EBLU. The guide panel GP supports the liquid crystal display panel LCP and uniformly maintains a distance between the liquid crystal display panel LCP and the optical film OPT. The top case TC is made of a metal material, for example, zinc coated steel and has a structure surrounding an upper surface and a side of the guide panel GP. The top case TC is fixed to at least one of the guide panel GP and the cover bottom CB using a hook or a screw.
It is preferable, but not required, that the light source LS uses a light emitting device (for example, a light emitting diode (LED)) having a high luminance using low electric power. The light source LS of the edge type backlight unit EBLU provides light for the light guide plate LG. The light source LS of the edge type backlight unit EBLU is positioned on the side of the liquid crystal display panel LCP. Namely, the light source LS is positioned corresponding to at least one side of the light guide plate LG and provides light for the side of the light guide plate LG.
The light guide plate LG has a rectangular cuboid panel shape having a face corresponding to an area of the liquid crystal display panel LCP. An upper surface of the light guide plate LG is positioned opposite the liquid crystal display panel LCP. The light guide plate LG receives light from the light source LS positioned on the side of the light guide plate LG and evenly diffuses and distributes the light inside the light guide plate LG, thereby functioning to induce light to the upper surface of the light guide plate LG, on which the liquid crystal display panel LCP is disposed.
It is not proper to use light induced to the liquid crystal display panel LCP by the light guide plate LG as backlight. For example, the light may not have a uniform luminance distribution throughout the entire area of the liquid crystal display panel LCP. Alternatively, the light may not be concentrated on the surface of the liquid crystal display panel LCP in a main observer direction. Thus, the light needs to be concentrated and diffused, so that the light is completely used as the backlight.
The optical film OPT is disposed between the light guide plate LG and the liquid crystal display panel LCP for the above-described functions. A structure of the optical film OPT according to the related art is described below with reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view illustrating a structure of an optical film including a diffusion film in a liquid crystal display according to a related art.
As shown in FIG. 3, an optical film OPT disposed at a lower part of the liquid crystal display LCP has a stack structure of generally widely used optical films. For example, the optical film OPT may have a structure, in which a lower prism sheet PRL, an upper prism sheet PRU, and a diffusion sheet DIF are sequentially stacked.
Triangular prism patterns are disposed on an upper surface of the lower prism sheet PRL. In particular, peaks and valleys of the triangular prism patterns are alternately disposed, and the peaks are arranged in parallel with one another in a first direction. The upper prism sheet PRU may have the same pattern as the lower prism sheet PRL. However, it is preferable, but not required, that triangular prism patterns of the upper prism sheet PRU are arranged in a second direction perpendicular to the first direction. Light emitted from the light guide plate LG passes through the lower prism sheet PRL and the upper prism sheet PRU and is concentrated to have a Gaussian distribution with respect to a normal line to the surface of the liquid crystal display LCP.
The diffusion sheet DIF evenly distributes light passing through the prism sheets PRL and PRU to the entire surface of the liquid crystal display panel LCP, thereby achieving a uniform luminance distribution. For example, in an edge type backlight unit, a side occupied by a light source may have a luminance greater than a side opposite the side. Further, in a direct type backlight unit, a formation portion of the light source may have a luminance greater than a portion around the light source. Namely, the diffusion sheet DIF causes a non-uniform luminance distribution of light over the entire surface of the liquid crystal display panel LCP to change a uniform luminance distribution. For a diffusion function, the diffusion sheet DIF may include beads BD distributed on an upper surface of the diffusion sheet DIF.
Light may be changed to a state suitable to use as the backlight by the prism sheets PRL and PRU and the diffusion sheet DIF. However, as light passes through the optical film OPT, a luminance of the light is reduced. This reduces an energy efficiency required to generate the backlight. In particular, the diffusion sheet DIF seriously affects a reduction in the luminance. In order to solve this, a dual brightness enhancement film has been proposed. FIG. 4 is a cross-sectional view illustrating a structure of an optical film including a dual brightness enhancement film in a liquid crystal display according to a related art.
A dual brightness enhancement film has a structure, in which a high refractive layer and a low refractive layer are stacked. Hence, the dual brightness enhancement film again reflects light, that is lost by a reflection, on its upper surface and prevents a reduction in the luminance. An optical film OPT shown in FIG. 4 has substantially the same structure as the optical film OPT shown in FIG. 3, except that a dual brightness enhancement film DBEF is used instead of the diffusion sheet DIF.
As described above, the related art optical film OPT has the structure, in which the components are sequentially stacked between the liquid crystal display panel LCP and the light guide plate LG. Namely, the upper prism sheet PRU is laid down on the lower prism sheet PRL. Thus, a predetermined air layer is formed between the upper prism sheet PRU and the lower prism sheet PRL. Because the air layer has a refractive index different from the prism sheets PRL and PRU, light passing through the air layer can be diffused.
The diffusion sheet DIF or the dual brightness enhancement film DBEF is laid down on the upper prism sheet PRU. Therefore, a predetermined air layer is formed between the upper prism sheet PRU and the diffusion sheet DIF or between the upper prism sheet PRU and the dual brightness enhancement film DBEF. As described above, light passing through the air layer can be further diffused.
However, the related art optical film OPT becomes thick because of its stack structure and is a barrier to a thin profile of the liquid crystal display. An ultra-thin profile of the liquid crystal display has been attempted by attaching the components of the optical film OPT. However, when the components of the optical film OPT are simply attached, the air layer is not formed. Therefore, a diffusion effect resulting from the air layer cannot be obtained, and the luminance distribution is not uniform. Further, moiré, rainbow mura, or hotspot fringe may be generated. Moisture penetrates between the peaks of the prism sheet by a capillary phenomenon, causing degradation in image quality. The backlight unit having the non-uniform luminance, the fringe generation, the moisture penetration, etc., was evaluated as unfit to provide the backlight. Thus, the ultra-thin profile of the related art liquid crystal display cannot be achieved.